The present invention pertains to the production of multilayer electrochromic devices (ECDs). More specifically, this invention pertains to a hybrid process for the production of ECDs utilizing plasma chemical vapor deposition (CVD) and at least one other deposition technique.
The phenomenon of chromogenic properties has been extensively studied because of its extreme importance in architectural and automotive markets. Chromogenic properties relate to a change in optical properties of a given material upon the application of a stimulus. Electrochromic materials undergo a reversible coloration with the application of an electric field or current, thus giving full control of the coloration to the user. The color change may be the result of the formation of color centers or alternatively an electrochemical reaction that produces a colored compound. The phenomenon of electrochromism in tungsten trioxide (WO3) was first reported by S. K. Deb in Applied Optics, Supplement 3, Electrophotography, (1969) page 192. Much has been written about the energy savings and increased comfort gained if large electrochromic devices (ECDs) were available as a feature of architectural and automotive windows.
A simple deposition of a thin film of the electrochromic material on a transparent substrate is not practical as an ECD. A functional ECD requires the sequential combination with other material thin films. A typical multilayer ECD consists of the following layers deposited sequentially on a transparent substrate such as glass or a thin polymer web:                1. (Optional) Substrate barrier layer: prevents migration of harmful substances from the substrate into the ECD.        2. Bottom electron conducting layer: provides a means to connect a power source/ground to the ECD.        3. Working electrode: the electrochromic material that reversibly changes color upon the reversible injection of ions and electrons.        4. Ion conducting layer: a layer that is both electron insulating and ion conducting. This section may need to be made of multiple materials or multiple layers for compatibility with the working electrode and ion storage electrode.        5. Ion storage electrode: stores the ions from the working electrode. It must have no color change or a complementary color change with the working electrode, i.e., when the working electrode is transparent the ion storage electrode must also be transparent, and when the working electrode is colored the ion storage electrode may or may not be colored.        6. Top electron conducting layer: provides a means to connect a power source/ground to the ECD.        7. (Optional) Top barrier layer: protects the ECD from contamination from the environment external to the device. This barrier layer may be a thin film or a laminated thick material, but must be transparent.        
When a current is applied, the ions move from the ion storage electrode, through the ion-conducting layer and are injected into the working electrode. By reversing the current direction the ions are removed from the working electrode and injected back into the ion storage electrode. The working electrode changes color upon the injection and removal of the ions. For example W(VI)O3 is transparent but if combined with a Li ion and electron it is converted to LiW(V)O3, which is highly colored.
The thickness of various layers required in a multilayer ECD structure is dependent upon the function of layers. For example, the thickness of top and bottom electrodes can vary from 200 to 500 nanometer (nm). The thickness of the ion conductive layer can vary from 50 to 300 nm. Likewise, the thickness of working and ion storage electrodes can vary from 200 to 700 nm.
Recently, different methods for depositing the sequential ECD layers on the substrate have been researched. Conventionally, ECDs are manufactured using a single deposition technology—primarily magnetron sputtering under vacuum. The deposition rate by sputtering process greatly depends upon the material that is being deposited. This method can be acceptable for easily sputtered or evaporated materials such as the material used for depositing electroconductive and ion conductive layers. It can be extremely slow for materials that are difficult to sputter or evaporate such as the materials used for depositing working and ion storage layers. Consequently, the sputtering process can be performed to produce ECDs on a large scale, but it becomes very difficult and time consuming to use the sputtering process for depositing working and ion storage electrode layers. For example, typical sputter deposition rates of a common working electrode material, WO3, are much less than one nm per second. Therefore, it is difficult to manufacture the entire multilayer ECD economically by using only vacuum sputter as the sole deposition method.
In the vacuum evaporation and sputtering deposition techniques, thin films of WO3, for example, are deposited in a vacuum environment from sources of W in an oxidizing atmosphere or WO3 in an inert or an oxidizing atmosphere. In the vacuum evaporation technique, the source material is heated to a vapor pressure sufficient to cause evaporation and condensation of the material onto a substrate. In the sputtering technique, the source material is converted to the vapor phase by positive ion bombardment. In both cases, thin film deposits of the WO3 are formed by vapor condensation on a substrate in the vacuum chamber.
U.S. patent application Ser. No. 2001/0031403 A1 describes a completely solid state ECD preferably deposited entirely by vacuum sputtering. The specification also discloses that other deposition techniques could be used or combined with vacuum sputtering. The vacuum techniques described are all physical deposition techniques (evaporation, reactive evaporation and reactive sputtering) and the “decomposition of precursors” techniques such as thermal pyrolysis and sol-gel. Thermal pyrolysis can be a vacuum or a non-vacuum technique. Sol-gel is, on the other hand, a non-vacuum, wet deposition technique.
The sol-gel deposition technique produces an oxide coating by depositing a colloidal solution onto a substrate. U.S. Pat. Nos. 5,659,417 and 5,699,192 both describe an ECD containing ion-conducting layer deposited by sol-gel techniques. While these patents describe forming all the layers by various sol-gel techniques, the examples also describe depositing the ion-conducting layer via sol-gel techniques, and depositing the other layers by reactive sputtering. However, these techniques suffer from inefficiencies related to alternating between vacuum and wet deposition techniques.
Plasma enhanced deposition techniques produce thin films by synthesizing reaction products from several ionized gaseous reactants under vacuum. In this context, plasma is an electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. Plasma enhanced deposition occurs when an electrical discharge in a low-pressure mixture of volatile reactants causes the formation of a variety of highly energetic species, e.g., atoms, metastables, radicals, and ions. These species then chemically interact to form stable deposits. In plasma enhanced chemical vapor deposition (PECVD), the power required to stimulate the gas-phase chemical reactions can be provided by radio-frequency electromagnetic radiation directed into the low pressure reaction chamber. Plasma enhanced CVD can in many cases provide deposition rates that are more than one order of magnitude higher than typically observed with vacuum sputtering even for the materials that are difficult to deposit by sputtering.
The PECVD method has been utilized to prepare electro-optically active transition metal oxides as disclosed in U.S. Pat. No. 4,687,560. The patent discloses thin films that would be useful as a working electrode and ion storage electrode. ECD structures are disclosed.
U.S. Pat. No. 6,156,395 discloses a PECVD method in the production of vanadium oxide thin-film layers. The patent discloses that the claimed deposition method and thin films could be used in an ECD. Vanadium-based films are one of the preferred compositions in an ECD for the ion storage layer.
U.S. patent application Ser. No. 2003/0156313 discloses an ECD, where the structure comprises at least one layer of electrochromic material and a layer of electronic insulating transparent ion-conducting solid electrolytic material, and where at least one of these layers is nanostructured. The reference discloses that the layers may be deposited by vacuum sputtering, PECVD, or vapor phase physical deposition techniques. Although the reference teaches that a nanostructured layer can comprise a plurality of layers deposited under different conditions, there is no suggestion of using a combination of PECVD and another deposition technique to provide a single layer within the ECD or different layers in the same ECD. Combinations of RF sputtering and ionic sputtering are the only examples given.
Accordingly, it is desired to provide a more efficient and less expensive method for manufacturing an ECD. It is further desired to provide such a method, which utilizes a combination of different deposition techniques to enhance the overall efficiency of the method.
All references cited herein are incorporated herein by reference in their entireties.